present invention relates to a charge transfer device and, more particularly, to a charge transfer device provided with an improved output circuit.
In recent years, charge transfer devices using charge transfer cells or devices (CTDs) have been employed in the video equipment, e.g., televisions, video disk players (VDPs), and video tape recorders (VTRs) etc. because the broad-band characteristic and the delay characteristic are both excellent. Especially in recent VTRs, high picture quality and frequency broad-band have been developed, and therefore charge transfer devices using CTDs have been widely employed.
With reference to FIG. 1, an explanation will be made in connection with a typical charge transfer device of the system in which the floating diffusion region (FD) system is applied to its output circuit to convert a signal charge transferred by charge transfer cells to a voltage signal to detect it as an output signal. A transfer unit of the charge transfer device is composed of charge transfer cells on which transfer electrodes 1, 2, . . . , 5 are arranged, respectively. A floating diffusion region 21 is provided in a manner that it is adjacent to the transfer electrode 5 of the final stage of the transfer unit. An impurity region 23 is further provided through a gate electrode 22 adjacent to the floating diffusion region 21. This impurity region 23 is connected to a reset power source 24 having a reset voltage V.sub.gg. Moreover, the above-mentioned floating diffusion region 21 is connected to a detection circuit 6. This detection circuit 16 is configured as a source follower circuit comprising a drive transistor 13, a power source 14 and a constant current source 15.
The operation of the charge transfer device shown in FIG. 1 will be now described in conjunction with the potential profile shown in FIG. 2.
It is now assumed in the charge reset mode that a transfer pulse .PHI..sub.1 applied to the transfer electrodes 1 and 2 and a transfer pulse .PHI..sub.3 applied to the transfer electrode 5 represent low level, whereas a transfer pulse .PHI..sub.2 applied to the transfer electrodes 3 and 4 and a reset pulse .PHI..sub.2 applied to the gate electrode 22 represent high level. Thus, as shown in FIG. 2(a), a signal charge Q.sub.s which has been transferred through the transfer unit is stored or accumulated below the transfer electrode 4 and the floating diffusion region 21 is reset to the reset voltage V.sub.gg.
When the operational mode shifts to the charge detection mode, the transfer pulses .PHI..sub.1 and .PHI..sub.3 become high level, and the transfer pulse .PHI..sub.2 and the reset pulse .PHI..sub.2 become low level. Accordingly, the signal charge Q.sub.s stored below the transfer electrode 4 is transferred to the floating diffusion region 21 as shown in FIG. 2(b). By the signal charge Q.sub.s, the floating diffusion region 21 produces a potential change .DELTA.V expressed as follows: EQU .DELTA.V=Q.sub.s .multidot.C.sub.o
where C.sub.o denotes an output capacity of the charge transfer cell. The potential change .DELTA.V of the floating diffusion region 21 is detected by the detection circuit 16 as an output signal.
Meanwhile, since there is a tendency that the power supply voltage of the circuit system in portable VTRs or the like is caused to be equal to a low voltage such as 5 volts, it is now strongly required for the charge transfer devices assembled therein as well to operate at a low voltage. On the other hand, it is required for the first stage of a video amplifier responsive to an output signal of the charge transfer device that the source follower of the detection circuit 16 operates with it having a good linearity.
Assuming now that a reset voltage, a power supply voltage of the power source 14 of the detection circuit 16, and a threshold voltage of the drive transistor 13 are represented by V.sub.gg, V.sub.DD and V.sub.th, respectively, the drive transistor 13 is required to operate in a saturated region in order that the source follower of the detection circuit 16 maintains linearity. To meet this, the following relationship must hold: EQU V.sub.gg -V.sub.th &lt;V.sub.DD.
In this instance, if the power supply voltage V.sub.DD is lowered owing to the requirement of allowing the power supply voltage to be reduced, it is required to lower the reset voltage V.sub.gg or to change the threshold voltage V.sub.th of the drive transistor 13. However, if the reset voltage V.sub.gg is lowered, the output dynamic range of the charge transfer cell becomes narrow. To avoid this, the reset voltage V.sub.gg is increased by a step-up circuit to widen the dynamic range. However, When such a countermeasure is taken, the linearity of the source follower of the detection circuit 16 becomes degraded in turn.
As just described above, the drawback with the conventional charge transfer device is that the output dynamic range of the charge transfer cell becomes narrow according as the power supply voltage lowers, or the linearity of the detection circuit for detecting an output signal becomes degraded.